1. Field of the Invention
The present invention is directed to a photometer for providing a digital indication of the light absorption properties of the test sample.
2. Description of the Prior Art
In known photometers a light beam is passed through a test sample in a glass cuvette or test cell and applied to a photoelectric sensor. The photoelectric sensor provides a voltage in accordance with the light absorbance of the test sample which is used for readout purposes. As the criteria for the light translucent properties of the test sample is the result of the light falling on the photoelectric sensor, the brightness qualities of the light source enter into the test results. With null balance, one may compensate for momentary brightness deviations but balance will be valid only for short times. Accordingly, the resulting comparison is affected in that every alteration of lamp brightness causes a migration of the reading from null. The intensity of the light source also varies rather strongly with stabilizing voltage so that exact measurement is not possible without continuous control and adjustment of the null point.
The purpose of the invention of the herein-described photometer is to provide, with minimal apparatus costs, continuous compensation for brightness variation of the illumination source. The purpose is further to provide, in addition to this brightness variation compensation, a time period which is directly proportional to the test results and which may be used to control a digital indicating or readout device.
To accomplish the foregoing purposes, the present invention provides a photometer for digitally indicating the light absorption properties of a test sample in a cuvette. The basis for the light absorption indication is the discharge time of a capacitor which is determined by the voltage difference between a light intensity responsive voltage produced by the application of a light beam to a comparison cell and a light intensity responsive voltage dependent on the light emerging from the measuring cell cuvette. The former voltage is used to charge the capacitor and the latter voltage terminates the discharge of the capacitor.
As criteria for the light transmission properties of the analyzed sample, the ratio of the quantity of the light applied to the sample to the quantity of light emerging from the sample will thus be employed. For this purpose a comparison light beam will be generated which is proportional to the intensity of the light rays applied to the cuvette. It is preferable in order not to diminish the useful intensity of the light rays which are applied to the analysis sample to divert only a small part of the light quantity in the generation of the comparison ray. By sufficiently high amplification with an amplifier, a signal which corresponds to the light intensity applied to the cuvette may be obtained. A measuring sensor, for example, a photoelectric sensor senses the light transmitted through the analysis sample. Electrically, with the help of the discharge characteristics of a capacitor, the quotient of both the value of the measuring light beam and value of the comparison light beam will be used in obtaining the results. This result is dependent on the quotient of the value of the measuring and comparison light beams. As the result is not dependent on the brightness of the light source not only does extremely good long term stability result but the burning time of the lamp is shorter and heat generation less than with conventional apparatus.
Through the discharge of a capacitor with an exponential discharge curve, the time t is provided as a function of the discharge. As this discharge period is logarithmically proportional to the value of the measuring and comparison light beams, it can be applied to the control of a digital counter. In the indication of the counter the extinction or absorbance E.sub.A is immediately available due to its logarithmic proportion to the time.
Other advantages are obtained by utilizing a step switch to provide variable resistance in parallel with the capacitor which is charged by the voltage proportional to the comparison light beam. One can thereby control the slope of the discharge curve and the time constant of the capacitor discharge period. A constant factor F can thus be adjusted which can serve to determine the concentration, i.e. a direct indication of concentration.
In another inventive aspect, the capacitor is about 10% overcharged. This advances the commencement of the discharge of the capacitor, i.e. the null point of the photometer. Advancement of the null point serves to obtain approximately similar proportions for the commencement and cessation of the discharge curve. Further, to this arrangement it is possible to alter the commencement of capacitor discharge which is also advantageous in controlling the null point.
The employment of comparators is also advantageous in the invention. These comparators compare the voltage of the test and comparison cells and provide a voltage proportional signal which is employed for the forming of time impulse t. The result is a simplified construction of the apparatus.
The partially transparent mirror used in the photometer of the present invention is employed in connection with an analog amplifier for important and unique advantages. The mirror needs to split only a signal of proportionally small intensity, which signal is then further amplified with an analog amplifier so that it corresponds to the intensity of the light beam applied to the cuvette. The major portion of the light of the light source is applied to the test sample to generate the light emerging from the test sample.
In a known way, through a backing device, a comparison light beam will be split before the cuvette with the test sample. The comparison light beam illuminates a photoelectric cell to provide a voltage which is amplified proportional to the intensity of the light beam applied to the cuvette. The test sample in the cuvette is illuminated and the intensity thereof related to the intensity of the comparison light beam. The light emerging from the cuvette does not contain the portion absorbed in the test sample. A voltage produced by the emerging light is led to an amplifier in a manner similar to that produced by the comparison light beam.